Changes in size and shape of synaptic connections after visual training: An ultrastructural approach of synaptic plasticity

The effect of visual training³⁰ on the synapses in the visual cortex of rabbits was quantitatively investigated. The number, the size and the fine structural organization of synaptic grids were analyzed in 0.5 μm E-PTA sections. The size of the pre- and postsynaptic terminals, the length of the apposition zones, the number of synaptic vesicles per terminal, the dimensions of the synaptic cleft and postsynaptic density and the frequency of mitochondria and spine apparatuses were studied in conventional OsO₄ sections.As compared to control animals, the synaptic population of visually trained rabbits exhibits 3 important changes:

1. (1) the number of complex grids has significantly increased; (perforated synapses according to refs. 6 and 24),

2. (2) the size of the synaptic grids has decreased. As is deduced from careful electron microscopic observations, this decrease in size is the result of a focalization of large grids.

3. (3) a significant increase in thickness of the postsynaptic density and a decrease in synaptic cleft is observed. The changes are most pronounced in deep layers (L IV and V) and are almost absent in superficial layers (L I).

On account of recent insights into the mechanism of synaptic transmission and of the intrinsic relation between the fine structural components of the synaptic junction and the process of transmitter release, the observed changes are interpreted as an expression of enhanced synaptic efficacy. It is further postulated that modification of synaptic efficacy must be considered as an important factor in the adaptation of the nervous system to changing environmental conditions and for the consolidation of memory traces.